The present invention relates generally to superconducting magnets, and more particularly to an assembly for venting cryogenic gases away from a superconducting magnet when gas pressure builds to a potentially dangerous level due to magnet quench or reversion to a non-superconductive state.
It is well known that magnets can be made superconductive by enclosing them within a pressure vessel, or cryostat, which contains a reservoir of liquid helium or some other cryogenic liquid. The extreme cold reduces the resistance in the coils of the magnet such that when a power source is initially connected to the coil to introduce a current flow through the coils, the current will continue to flow through the coils due to the negligible resistance even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (hereinafter xe2x80x9cMRIxe2x80x9d).
During normal superconducting operation of the magnet, the cryostat must be a sealed or closed system so as to prevent leakage of helium gas from the cryostat and depletion of liquid helium from the reservoir. In the event of an undesired magnet quench or reversion of the magnet to a non-superconductive state, however, a rapid and potentially dangerous helium gas high pressure buildup in the cryostat requires pressure relief through rapid venting of the gas to the atmosphere outside the superconducting magnet. A replaceable burst disk may be interposed within a magnet vent assembly, which disk is designed to rupture at a predetermined pressure thereby opening the cryostat to an atmospheric vent. The atmospheric vent may be a vent stack which extends from the roof of a building or from the roof of a motor vehicle which is used to transport a portable MRI system contained within it.
Once ruptured, the burst disk must be replaced. Replacement of the burst disk and resealing of the vent assembly is necessary to prevent leakage of gaseous helium or other cryogen during normal superconducting operation of the magnet. The problem of gas leakage is most significant in superconducting magnets which recondense helium gas back to liquid helium. These are often referred to as zero boil-off magnets which are designed to minimize the difficulties encountered in shipping and storing the necessary reserve supply of liquid helium at cryogenic temperatures and the related problem of periodically transferring a portion of the liquid helium in the storage reservoir to the liquid helium supply in the MRI superconducting magnet. A helium gas leak within the venting assembly, including a leak at the level of the burst disk, will result in the need to continuously add or replenish the liquid helium thus defeating the goal and advantage of a zero boil-off magnet.
It has, however, proven difficult as a practical matter to properly seal the vent assembly, yet provide for ready replacement of the burst disk and resealing of the vent assembly following a high pressure rupture of the burst disk. The installation and removal of a pressure vessel burst disk has also proven to be a time consuming and labor intensive task. It has further been determined to be desirable to prevent the burst disk, which is constructed of a carbon graphite material, from being a structure carrying member of external forces due to the weight of other components which are typically used to construct the magnet vent assembly.
What is needed is a design for an improved burst disk assembly for a superconducting magnet which provides for ready and uncomplicated replacement of the burst disk after rupture, which preserves integrity of the vent sealing during normal operation of the magnet and which relieves the burst disk itself of any dynamic loading.
A burst disk assembly is provided for use with a superconducting magnet system having a cryogenic gas vent. The gas vent is attached at one end to the cryogenic liquid reservoir of the magnet and is attached at the other end to an atmospheric exhaust vent. Cryogenic gas is vented away from the liquid reservoir to the atmosphere outside the magnet in the event of a rapid and potentially dangerous high pressure gas buildup. The burst disk assembly includes a composite design burst disk which is interposed within the vent to seal and effectively block the vent during normal operation of the superconducting magnet. The burst disk is selected to rupture, or burst, in the event of the buildup of cryogenic gas pressure which exceeds a predetermined amount. The burst disk is a composite design constructed of a solid carbon graphite center surrounded by an integrally formed stainless steel ring. A captured burst disk flange is provided which insures proper placement and seating of the burst disk during replacement and which supports the burst disk so as to prevent it from falling or dropping out of the assembly during replacement of a ruptured burst disk. A deformable gasket contiguous to the burst disk seals the burst disk to the captured burst disk flange when the disk and the flange are pressed together. Attachment bolts are used to provide for even application of pressure along the face of the burst disk and the captured burst disk flange. Hexagonal head bold heads are captured in slightly larger hexagonal patterns which are cast into the vent assembly components thereby allowing the technician who removes or installs the burst disk to tighten or loosen the attachment bolts using a single wrench. These features make the burst disk assembly more xe2x80x9cuser friendlyxe2x80x9d for a cartridge type installation. The captured burst disk flange also prevents the burst disk from being a structure carrying member of external forces due to the weight of other components of the vent assembly and relieves the burst disk of any dynamic loading.